Devices for inductive sealing comprise essentially a high-frequency generator (HF-Generator) and an induction coil (or briefly: coil), which is arranged in a sealing head. The coil can be constructed flat or tunnel-shaped and in addition can be surrounded by a ferrite enclosure, which concentrates the electromagnetic field in the application direction. The coil is normally connected into a series resonant circuit, which is driven at close to the resonance frequency, in order to generate current overshoots in the coil.
The sealing head is normally arranged above a conveying device, which enables the containers to be sealed or packages to be conveyed through under the coil. The containers are closed by means of a lid that can be screwed on or snapped into place, which contains a metal foil and an adjacent plastic sealing film. On account of the induction coil, eddy currents are generated in the metal foil, which heat up the metal foil. Due to the contact between the metal foil and the sealing film, the sealing film is also heated up and is thereby fused with the container opening. Tunnel-shaped induction coils have the advantage over flat coils that even containers with a large distance between the metal foil and the top edge of the lid can be sealed, as the coil acts on the metal foil from the side.
It is known from EP 0 842 854 B1 (DE 697 04 191 T2) to configure the geometry of a sealing head with a tunnel-shaped induction coil in such a way that container openings of various widths can be sealed. In this device there is a step-shaped opening in the sealing head, which in operation receives container lids of different widths in its different-sized opening sections. A disadvantage of the device described in EP 0 842 854 B1 however is the fact that it is only optimally matched to container lids of pre-specified size, and that the action of the electromagnetic field of the induction coil on the containers takes place mainly from above, so that containers with tall lids, i.e. with a large distance between the top edge of the lid and the internal metal foil can only be poorly sealed.